It is well known in the art that substrates experience temperature variations during an automated soldering process due to varying component masses. Specific oven profiles control the temperature variations in high mass areas by increasing the heating time and in low mass areas by decreasing the heating time. If the oven profile drifts, the component mass changes, or the emissivity of the substrate changes, the automated soldering process may yield unacceptable results. To avoid having to rework or scrap product, the soldering process is periodically verified by verification methods such as visual inspections or thermal profile runs.
The visual inspection method verifies the soldering process by periodically sampling the substrates and visually inspecting the solder connections. The visual inspection step is usually performed after soldering process is complete and checks for solder bridges, solder balls and misheated solder connections. This method has limited usefulness because it samples only a small percentage of substrates, requires an extra production step, and does not provide individual substrate test data.
The thermocouple profile run method periodically verifies the soldering process by measuring the oven profile temperatures. This method runs a test substrate instrumented with thermocouples through the automated soldering process. The test substrate records the oven profile temperatures and determines if the oven profile has drifted. This method has limited usefulness because it interrupts the soldering process, requires an extra production step, and does not provide individual substrate test data.
Another verification method employs automated x-ray equipment to inspect the soldering process by inspecting the solder connections. This method inspects the resulting solder connections and detects soldering defects such as porosity, solder bridges and solder balls. This method inspects each substrate without additional production steps; however, it only measures the amount and location of solder, not whether the solder has reflowed properly.
A need therefore exists for a solder verification method that will be an integral part of the soldering process, does not require additional production steps, inspects and produces test results for each substrate, and verifies proper solder reflow.